Process for preparing selenium tellurium alloys

ABSTRACT

A SELENIUM-TELLURIUM ALLOY SUITABLE FOR ELECTROPHOTOGRAPHIC PHOTOSENITIVE MEMBER IS PREPARED BY HEATING A MIXTURE OF SELENIUM AND TELLURIUM CONTAINING 1-25% BY WEIGHT OF TELLURIUM TO A TEMPERATIVE NOT LOWER THAN 350*C. TO MELT THE MIXTURE, COOLING GRADUALLY THE MOLTEN SELENIUM AND TELLURIUM TO AROUND THE MELTING POINT OF THE SELENIUM-TELLURIUM ALLOY AT A RATE NOT HIGHER THAN 100*C./HR. AND THEN QUENCHING TO ROOM TEMPERATURE WITHIN 10 MINUTES.

March 1973 NOBUO KITAJIMA- ETAL 3,723,105

PROCESS FOR PREPARING SELENIUM-TELLURIUM ALLOYS Filed Sept. 13, 1971 SURFACE POTENTlAL (V) United States Patent U.S. Cl. 75--134 H 1 Claim ABSTRACT OF THE DISCLOSURE A selenium-tellurium alloy suitable for electrophotographic photosensitive member is prepared by heating a mixture of selenium and tellurium containing 125% by weight of tellurium to a temperative not lower than 350 C. to melt the mixture, cooling gradually the molten selenium and tellurium to around the melting point of the selenium-tellurium alloy at a rate not higher than 100 C./hr. and then quenching to room temperature within 10 minutes.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to a process for preparing selenium-tellurium alloys having excellent electrophotographic photosensitive characteristics and high durability.

Description of the prior art It is well known that selenium-tellurium alloys are used for an electrophotographic photosensitive member. The photosensitivity has been improved by various methods such as methods disclosed in U.S. Pat. No. 2,745,327 and U.S. Pat. No. 2,803,541 in which contents of selenium and tellurium are changed, methods disclosed in J. C. Schottmiller, D. L. Bauman and C. Wood: Journal of Applied Physics, vol. 39, No. 3, pp. 1663-9 (1968) in which an element other than selenium and tellurium is added to the alloy, and methods disclosed in Taul. H. Keck: Journal of Optical Society of America, vol. 42, No. 4, pp. 221- (1952) and Taul. H. Keck: Journal of Optical Society of America, vol. 41, No. 1, pp. 53-55 (1951) in which the depositing conditions such as a temperature of deposit base and depositing time are appropriately controlled upon preparing the photosensitive member.

The above mentioned prior arts fails to produce selenium-tellurium alloys having satisfactory electrophotographic properties. Particularly, it has been very diflicult to obtain selenium-tellurium alloys free from residual potential which causes fog.

SUMMARY OF THE INVENTION for preparing selenium-tellurium alloys having a good electrophotographic characteristics.

It is another object of this invention to provide a process for preparing selenium-tellurium alloys of good photosensitivity and high durability for electrophotography.

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It is a further object of this invention to provide a process for preparing selenium-tellurium alloys forming no residual potential and capable of producing electrostatic latent images of high contrast when used in electrophotographic processes.

It is still another object of this invention to provide a process for producing selenium-tellurium alloys having photosensitive characteristics suitable for electrophotographic processes by controlling temperature conditions in the preparation.

BRIEF DESCRIPTION OF THE DRAWING The figure is a graph for explaining the definition of the sensitivity used in this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The mixture of selenium and tellurium used in this invention contains 1-25% by weight of tellurium, preferred with 520% by weight, and balance of selenium. When the contents of tellurium is less than 1% by weight, dark resistance increases to reduce the sensitivity while dark resistance becomes too low to use as an electrophotographic photosensitive member when the contents of tellurium is higher than 25% by weight.

The cooling procedure in this invention comprises two steps i.e. gradual cooling and quenching. The gradual cooling step is carried out by cooling the melt of the selenium and tellurium at a temperature not lower than 350 C. to around the melting point of the seleniumtellurium alloys at a cooling rate of not higher than C./hr. The quenching step is carried out by quenching the selenium and tellurium composition at around the melting point to room temperature within 10 minutes. It is preferable to quench the alloys in an instant.

The mechanism of formation of the selenium-tellurium alloy of excellent electrophotographic characteristics is not yet fully understood, but it is believed as follows.

The gradual cooling step produces a relatively long binding chain of selenium-tellurium in a hexagonal structure and the quenching step freeze the thermally and chemically stable binding chain group in the alloy. As the result, thermal formation of ring-like or monoclinic structure of selenium-tellurium chain showing undesirable photosensible characteristics is inhibited and thereby deterioration of photosensitive characteristics of the selenium-tellurum alloy is prevented.

There is a prior art for producing selenium-tellurium alloys which comprises mixing powdered selenium and powdered tellurium of high purity, melting under vacuum and quenching the melt by throwing into water. In this prior art, it is believed that the hexagonal structure is not sufliciently formed and the relatively unstable binding chain group is frozen to produce a selenium-tellurium alloy having undesirable photosensitive characteristics.

There is another prior art for producing selenium-tellurium alloys which comprises mixing powdered selenium and powdered tellurium of high purity, melting under vacuum and cooling gradually to room temperature by allowing to stand. In this case, it is believed that the gradual cooling at the temperature range below the melting point of the selenium-tellurium mixture results in formation of binding chain of selenium and tellurium in a form of ring-like or monoclinic structure which deteriorates photosensitive characteristics.

The selenium-tellurium alloys obtained according to the present invention may be formed as a photosensitive layer on a support by a conventional vapor depositing method to produce a photosensitive member for electrophotographic processes. The photosensitive member may be that of three layer structure, that is, an insulating layer may overlie the selenium-tellurium alloy photosenstive layer.

The following examples are given for purposes of illustration and not by way of limitation.

EXAMPLE 1 (1) In a Pyrex glass vessel having a softening point of about 600 C. were placed selenium particles (purity, 99.99%) having radius of about 1.5 mm. and powdered tellurium (120-180 mesh) in a mixing ratio of 85 to by weight to fill /2 volume of the vessel. The vessel was brought to a pressure of about 5 10 torr and the upper end of the vessel was sealed, and then the sealed vessel was placed in a stainless steel container. Aluminum oxide wool was packed in the stainless steel container so as to prevent the Pyrex glass vessel from contacting with the stainless steel container. The stainless container was, then, covered with a lid, placed in an electric furnace, and heated. When the temperature of the furnace was about 550 C., the stainless steel container was vibrated for about 2 hours so as to agitate the melt in the Pyrex glass vessel. The heating was stopped, cooled gradually at a rate of about 100 C./hr. to about 260 C. and then thrown into water to quench the melt. The seleniumtellurium alloy thus produced is hereinafter called A.

(2) A stainless steel container containing a melt obtained by heating at about 550 C. for about 2 hours with stirring in a way similar to item (1) above was directly thrown into water for quenching. The resulting seleniumtellurium alloy is hereinafter called B.

(3) A melt obtained by heating at about 550 C. for about 2 hours in a way similar to item 1) above was cooled to room temperature by allowing to stand. The resuiting selenium-tellurium alloy is hereinafter called C.

The preparation procedures in items (2) and (3) are prior arts which are given here for comparing with the present invention.

Each of the above-mentioned three kinds of selenium tellurium alloy was vapor-deposited on a conductive base plate at a pressure of about 2 10 torr with a distance of cm. between the base plate and the melting boat containing the selenium-tellurium alloy at the same base plate temperature for the same vapor-depositing time to produce an electrophotographic photosensitive plate. There may be used various metals and conductive glasses, but a nickel-plated brass of about 1 mm. thick was employed here.

Photosensitive members obtained by a process as mentioned above were subjected to an electrophotographic process comprising uniform charging and exposure and an electrophotometer was used for measuring characteristics of the photosensitive member.

When a xerographic plate was positively charged (Carlson process) TABLE 1 Base Thickness plate Depositof photo- Sensi- Residual temperaing time, sensitive tivity, potential, ture, C. min. layer, t lux. sec. v.

The sensitivity is defined as an exposure amount when a dark decay potential a and a light decay potential b satisfy the relation:

zero so that any fog is not observed and a clear image of high contrast is obtained.

EXAMPLE 2 The procedure of Example 1 above was followed except that the ratio of selenium to tellurium by weight was 95:5 or :20 in place of :15 to obtain alloys A B and C or alloys A B and C The resulting alloy was used as a photoconductive layer to form a photosensitive plate. The characteristics of the photosensitive plate was measured by an electrophotometer. The result is shown in Table 2 and Table 3.

TABLE 2 [Weight ratio of selenium to tellurium, :5]

Base Thickness plate Depositof photo- Sensi- Residual temperaing time, sensitive tivity, potential, ture, 0 min. layer, a lux. sec. v,

TABLE 3 [Weight ratio of selenium to tellurium, 80:20]

Base Thickness plate Depositof photo- Sensi- Residual temperaing time, sensitive tivity, potential, ture, C. min. layer, lux. sec v EXAMPLE 3 Selenium-tellurium alloy A, B or C produced in Example 1 was vapor-deposited under vacuum on a nickelplated conductive base plate at a base plate temperature of 65 C. and further a polyester film of 25p. thick was adhered with an epoxy resin to the selenium-tellurium alloy deposit layer to product a photosensitive member of three-layer structure. The resulting photosensitive plate was given a uniform surface potential of 2.2 kv. by a primary corona charging and then was subjected to an AC. corona charging of 7.5 kv. simultaneously with projecting a light image at an exposure amount of 6 lux. sec. Further, a blanket radiation was applied to the surface of the photosensitive member (cf. Japanese patent publication No. 23910/ 1967). The dark decoy potential and the light decay potential were measured and the result is shown in Table 4 below.

TABLE 4 Dark surface Light surface Electrostatic potential, v. potential, v. contrast, v.

As is clear from Table 4, a photosensitive member using the alloy A shows an electrostatic contrast far higher than that of the alloy B or alloy C.

EXAMPLE 4 In a way similar to Example 3, a photosensitive member was given a surface potential of 2.2 kv. by a primary charging, subjected to projection of a light and dark pattern at an exposure amount of 6 lux. sec. simultaneously with applying a secondary corona charging of +7 kv. and then subjected to a blanket irradiation to produce an electrostatic latent image (cf. Japanese patent publication No. 24748/1968).

The dark surface potential and the light surface potential were measured. The result is shown in Table 5 below.

As is clear from the above table, the photosensitive member using selenium-tellurium alloy A shows an electrostatic contrast far higher than that using selenium-tellurium alloy B or C.

A photosensitive member obtained by vapor-depositing the selenium-tellurium alloy produced by the present invention is particularly suitable for electrophotographic processes since the residual potential is almost zero and a high electrostatic contrast can be obtained and thereby a clear and fogless image is produced.

What is claimed is:

1. A process for preparing a selenium-tellurium alloy which comprises heating a mixture of selenium and tellurium containing 125% by weight of tellurium to a temperature not lower than 350 C. to melt the mixture, cooling gradually the molten selenium and tellurium to around the melting point of the selenium-tellurium alloy at a rate not higher than 100 C./hr. and then quenching to room temperature within 10 minutes.

References Cited UNITED STATES PATENTS 2,745,327 5/1956 Mengali 961.5 2,803,541 8/1957 Paris 961.5 3,041,166 6/1962 Bardeen 96-1.5 3,655,377 4/1972 Sechak 961.5

CHARLES N. LOVELL, Primary Examiner E. L. WEISE, Assistant Examiner US. Cl. X.R. 961.5; 252-501 

